1. Field of the Invention
The invention relates to a method of manufacturing a LIGA mold, and more particularly to a method of manufacturing a LIGA mold by backside exposure.
2. Description of the Related Art
LIGA is the abbreviation of German “Lithographie, Galvanoformung und Abformung”, which means lithography, electroplating and molding. The LIGA process uses a mask and the X-ray having high linearity but requiring synchronous radiation to manufacture a micro structure having a pattern with a high aspect ratio (e.g., 50:1 or larger), wherein the fabricated micro structure may be an end product or a mold for proceeding the molding process such as hot embossing.
FIGS. 1A to 1C show conventional LIGA processes using the X-ray in the case of using a negative photoresist (e.g., SU-8). As shown in FIG. 1A, a seed layer 120 is deposited on a substrate 110. Then, a photoresist layer 130 is applied onto the seed layer 120 by way of spin coating. Next, a mask layer 140 is provided or formed on the photoresist layer 130. Then, an X-ray source 150 for illumination is provided to form the photoresist layer 130 with the patterned structure of FIG. 1A.
Then, as shown in FIG 1B, a metal layer 160 is formed on the seed layer 120 by way of plating. Next, the photoresist layer 130, the seed layer 120 and the substrate 110 are removed to remain the metal layer 160 as a mold. The mold can be used to perform the hot embossing process. Due to the excellent optical property of the X-ray, a sidewall of the patterned structure formed on the photoresist layer 130 is substantially vertical. In addition, the surface of the sidewall of the formed photoresist layer 130 is very smooth such that the surface of the formed mold is very smooth, which is advantageous to the de-molding procedure of the hot embossing process and is thus advantageous to the mass production. However, the X-ray source cannot be easily obtained and a synchronous radiation apparatus is needed, so the cost of the LIGA process using the X-ray cannot be effectively reduced.
FIGS. 2A to 2C show conventional LIGA processes using the ultra-violet light. As shown in FIGS. 2A to 2C, the photoresist layer 130, which is large in top and small in bottom, is formed when the ultra-violet light source 150′ is used to replace the X-ray source of FIG. 1A by way of front side exposure due to the Fresnel diffraction effect of the light and the absorption property of the photoresist. That is, an upper area 132 of the photoresist layer 130 is larger than a lower area 134 thereof. Thus, the manufactured mold 160 is disadvantageous to the de-molding procedure of the hot embossing process.
Consequently, it is an important subject of the invention to provide a method of forming a LIGA mold, which is advantageous to the de-molding procedure of the hot embossing process, by LIGA processes using the ultra-violet light.